The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to separating the NMR signal contributions from a plurality of different species having different chemical shifts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Magnetic resonance imaging (MRI) is a medical imaging modality that offers remarkable image contrast between soft tissues such as fat and muscle. While this soft tissue contrast is typically the hallmark of MRI, the substantially bright signal attributed to fat often causes difficulties when imaging regions of the body that may be obscured by fat containing tissues. This can impair clinical diagnoses, however, so methods for separating the MR signal from water and fat were developed.
Conventional fat suppression or water-fat decomposition methods model fat as a single resonant frequency at approximately 3.5 ppm (210 Hz at a field strength of 1.5 Tesla and 420 Hz at a magnetic field strength of 3.0 Tesla) away from the water resonant frequency. Exemplary methods of conventional fat suppression include spectral saturation (“FatSat”), and chemical-shift based multipoint Dixon methods. Recently, a new method known as IDEAL was developed for imaging spin species such as fat and water. As described in U.S. Pat. No. 6,856,134 issued on Feb. 15, 2005 and entitled “Magnetic Resonance Imaging With Fat-Water Signal Separation”, the IDEAL method employs pulse sequences to acquire multiple images at different echo times (TE) and an iterative, linear least squares approach to estimate the separate water and fat signal components. However, this method also models the fat signal as having one resonant frequency, as do all other reliable Dixon methods.
Fat has a complex spectral profile that includes multiple resonant frequencies. To exemplify this point, reference is made to FIG. 1, where a more accurate model of a fat resonant frequency spectrum is shown that includes six resonant frequencies. At a magnetic field strength of 1.5 Tesla, the fat spectrum has, relative to the water resonant frequency: one peak at −47 Hz, one at 23 Hz, one at 117 Hz, one at 159 Hz, one at 210 Hz, and one at 236 Hz. Conventionally, it is only the 210 Hz fat peak that is targeted in fat suppression methods and modeled in water-fat decomposition methods; however, this leads to undesired effects, especially when performing quantitative studies.
One such undesired effect of treating the fat signal as having a single resonant frequency results from the water and fat signals being incompletely separated. As a result of this incomplete separation, a baseline level of signal is manifested within adipose tissue on the separated water images. This effect occurs primarily because the fat spectral peak at −47 Hz is relatively close to the water resonant frequency and contributes around 10-20% of the overall signal from fat. To a lesser extent, even other spectral peaks such as the 117 Hz and 159 Hz side peak interfere with the water resonant frequency, depending on the sample times used for water-fat separation. While this undesired effect may be tolerable for some qualitative imaging studies, the incomplete suppression of fat reduces the desired contrast between water and adipose tissue in decomposed water images, an effect that can make the visualization of abnormalities difficult. For example, poor water-fat contrast can result in a radiologist overlooking tumors in the vertebral bodies.
In recent years, studies that rely on the quantification of fat have grown in interest. Inaccurate quantification of fat can therefore confound clinical diagnoses such as fatty infiltration of the liver. Another undesired effect from modeling the fat signal as having a single resonant peak presents a significant problem for T*2 estimation in the presence of fat since signal from the multiple fat spectral peaks can simulate faster than normal T*2 decay. As a result of this effect, signal from fat does not follow a monoexponential decay. Therefore, conventional T*2 estimation methods that model fat as having a single resonant peak produce underestimations of T*2, confounding quantitative studies where an accurate estimation of T*2 is required.